Resonant frequency tuning of micromachined mirror assembly

ABSTRACT

Embodiments of the disclosure provide a micromachined mirror assembly. The micromachined mirror assembly includes a micro mirror, a first suspended beam, a second suspended beam, a first actuator, and a second actuator. The micro mirror is configured to tilt around an axis. The first suspended beam and second suspended beam each is mechanically coupled to the micro mirror along the axis. The first actuator is mechanically coupled to the first suspended beam and configured to apply a first torsional stress around the axis to the first suspended beam. The second actuator is mechanically coupled to the second suspended beam and configured to apply a second torsional stress around the axis to the second suspended beam. the first torsional stress and second torsional stress have a magnitude difference.

TECHNICAL FIELD

The present disclosure relates to a micromachined mirror assembly, andmore particularly to, a micromachined mirror assembly used in a scannerfor light detection and ranging (LiDAR).

BACKGROUND

LiDAR systems have been widely used in autonomous driving and producinghigh-definition maps. For example, LiDAR systems measure distance to atarget by illuminating the target with pulsed laser light and measuringthe reflected pulses with a sensor. Differences in laser return timesand wavelengths can then be used to make digital three-dimensional (3-D)representations of the target. The laser light used for LiDAR scan maybe ultraviolet, visible, or near infrared. Because using a narrow laserbeam as the incident light from the scanner can map physical featureswith very high resolution, a LiDAR system is particularly suitable forapplications such as high-definition map surveys.

The scanner of a LiDAR system includes a mirror that can be moved (e.g.,rotated) by actuators to reflect (and steer) incident laser beams from alaser source towards a pre-determined angle. The mirror can be a single,or an array of micromachined mirror assembly(s) made by semiconductormaterials using microelectromechanical system (MEMS) technologies. Inorder to maximize the deflection angle of the micromachined mirrorassemblies for a given voltage, they are operated in their resonantfrequency. However, resonant frequency can shift due to thermalexpansion when temperature changes. Also, there may be inherent processvariations during fabrication of microstructures. Thus, achieving thetarget resonant frequency becomes especially important when multiplemicro mirrors need to be synchronized to operate at the same resonantfrequency.

Embodiments of the disclosure address the above problems by an improvedmicromachined mirror assembly in a scanner for LiDAR.

SUMMARY

Embodiments of the disclosure provide a micromachined mirror assembly.The micromachined mirror assembly includes a micro mirror, a firstsuspended beam, a second suspended beam, a first actuator, and a secondactuator. The micro mirror is configured to tilt around an axis. Thefirst suspended beam and second suspended beam each is mechanicallycoupled to the micro mirror along the axis. The first actuator ismechanically coupled to the first suspended beam and configured to applya first torsional stress around the axis to the first suspended beam.The second actuator is mechanically coupled to the second suspended beamand configured to apply a second torsional stress around the axis to thesecond suspended beam. The first torsional stress and second torsionalstress have a magnitude difference.

Embodiments of the disclosure also provide another micromachined mirrorassembly. The micromachined mirror assembly includes a micro mirror, afirst suspended beam, a second suspended beam, and at least onetensional actuator. The micro mirror is configured to tilt around anaxis. The first suspended beam and second suspended beam each ismechanically coupled to the micro mirror along the axis. The at leastone tensional actuator is mechanically coupled to an end of at least oneof the first and second suspended beams and configured to apply atensional stress along the axis to the at least one of the first andsecond suspended beams.

Embodiments of the disclosure also provide a method for driving amicromachined mirror assembly. A resonant frequency of the micromachinedmirror assembly is set at an initial value. A tensional stress isapplied along an axis of the micromachined mirror assembly to increasethe resonant frequency to a first operational value greater than theinitial value during operation of the micromachined mirror assembly.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of an exemplary vehicle equippedwith a LiDAR system, according to embodiments of the disclosure.

FIG. 2 illustrates a block diagram of an exemplary LiDAR system having atransmitter with a scanner, according to embodiments of the disclosure.

FIG. 3A illustrates a schematic diagram of an exemplary micromachinedmirror assembly, according to embodiments of the disclosure.

FIG. 3B illustrates a top perspective view of the exemplarymicromachined mirror assembly in FIG. 3A, according to embodiments ofthe disclosure.

FIG. 4A illustrates a waveform of an exemplary voltage signal applied tothe actuators of a micromachined mirror assembly, according toembodiments of the disclosure.

FIG. 4B illustrates a waveform of another exemplary voltage signalapplied to the actuators of a micromachined mirror assembly, accordingto embodiments of the disclosure.

FIG. 5 illustrates a schematic diagram of another exemplarymicromachined mirror assembly, according to embodiments of thedisclosure.

FIG. 6A illustrates a schematic diagram of a design of the exemplarymicromachined mirror assembly in FIG. 5, according to embodiments of thedisclosure.

FIG. 6B illustrates a schematic diagram of another design of theexemplary micromachined mirror assembly in FIG. 5, according toembodiments of the disclosure.

FIG. 7 illustrates a flow chart of an exemplary method for driving amicromachined mirror assembly, according to embodiments of thedisclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the exemplary embodiments,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

FIG. 1 illustrates a schematic diagram of an exemplary vehicle 100equipped with a LiDAR system 102, according to embodiments of thedisclosure. Consistent with some embodiments, vehicle 100 may be asurvey vehicle configured for acquiring data for constructing ahigh-definition map or 3-D buildings and city modeling.

As illustrated in FIG. 1, vehicle 100 may be equipped with LiDAR system102 mounted to a body 104 via a mounting structure 108. Mountingstructure 108 may be an electro-mechanical device installed or otherwiseattached to body 104 of vehicle 100. In some embodiments of the presentdisclosure, mounting structure 108 may use screws, adhesives, or anothermounting mechanism. Vehicle 100 may be additionally equipped with asensor 110 inside or outside body 104 using any suitable mountingmechanisms. Sensor 110 may include sensors used in a navigation unit,such as a Global Positioning System (GPS) receiver and one or moreInertial Measurement Unit (IMU) sensors. It is contemplated that themanners in which LiDAR system 102 or sensor 110 can be equipped onvehicle 100 are not limited by the example shown in FIG. 1 and may bemodified depending on the types of LiDAR system 102 and sensor 110and/or vehicle 100 to achieve desirable 3-D sensing performance.

Consistent with some embodiments, LiDAR system 102 and sensor 110 may beconfigured to capture data as vehicle 100 moves along a trajectory. Forexample, a transmitter of LiDAR system 102 is configured to scan thesurrounding and acquire point clouds. LiDAR system 102 measures distanceto a target by illuminating the target with pulsed laser beam andmeasuring the reflected pulses with a receiver. The laser beam used forLiDAR system 102 may be ultraviolet, visible, or near infrared. In someembodiments of the present disclosure, LiDAR system 102 may capturepoint clouds. As vehicle 100 moves along the trajectory, LiDAR system102 may continuously capture data. Each set of scene data captured at acertain time range is known as a data frame.

FIG. 2 illustrates a block diagram of an exemplary LiDAR system 102having a transmitter 202 with a scanner 210, according to embodiments ofthe disclosure. LiDAR system 102 may include transmitter 202 and areceiver 204. Transmitter 202 may emit laser beams within a scan angle.Transmitter 202 may include one or more laser sources 206 and a scanner210. As described below in detail, scanner 210 may include amicromachined mirror assembly (not shown) having a resonant frequencythat can be tuned during the operation of the micromachined mirrorassembly.

As part of LiDAR system 102, transmitter 202 can sequentially emit astream of pulsed laser beams in different directions within its scanangle, as illustrated in FIG. 2. Laser source 206 may be configured toprovide a laser beam 207 (referred to herein as “native laser beam”) ina respective incident direction to scanner 210. In some embodiments ofthe present disclosure, laser source 206 may generate a pulsed laserbeam in the ultraviolet, visible, or near infrared wavelength range.

In some embodiments of the present disclosure, laser source 206 is apulsed laser diode (PLD). A PLD may be a semiconductor device similar toa light-emitting diode (LED) in which the laser beam is created at thediode's junction. In some embodiments of the present disclosure, a PLDincludes a PIN diode in which the active region is in the intrinsic (I)region, and the carriers (electrons and holes) are pumped into theactive region from the N and P regions, respectively. Depending on thesemiconductor materials, the wavelength of incident laser beam 207provided by a PLD may be smaller than 1,100 nm, such as 405 nm, between445 nm and 465 between 510 nm and 525 nm, 532 nm, 635 nm, between 650 nm660 nm, 670 nm, 760 nm, 785 nm, 808 nm, or 848 nm.

Scanner 210 may be configured to emit a laser beam 209 to an object 212in a first direction. Object 212 may be made of a wide range ofmaterials including, for example, non-metallic objects, rocks, rain,chemical compounds, aerosols, clouds and even single molecules. Thewavelength of laser beam 209 may vary based on the composition of object212. At each time point during the scan, scanner 210 may emit laser beam209 to object 212 in a direction within the scan angle by rotating themicromachined mirror assembly as the incident angle of incident laserbeam 207 may be fixed. In some embodiments of the present disclosure,scanner 210 may also include optical components (e.g., lenses, mirrors)that can focus pulsed laser light into a narrow laser beam to increasethe scan resolution and range of object 212.

As part of LiDAR system 102, receiver 204 may be configured to detect areturned laser beam 211 returned from object 212 in a differentdirection. Receiver 204 can collect laser beams returned from object 212and output electrical signal reflecting the intensity of the returnedlaser beams. Upon contact, laser light can be reflected by object 212via backscattering, such as Rayleigh scattering, Mie scattering, Ramanscattering, and fluorescence. As illustrated in FIG. 2, receiver 204 mayinclude a lens 214 and a photodetector 216. Lens 214 may be configuredto collect light from a respective direction in its field of view (FOV).At each time point during the scan, returned laser beam 211 may becollected by lens 214. Returned laser beam 211 may be returned fromobject 212 and have the same wavelength as laser beam 209.

Photodetector 216 may be configured to detect returned laser beam 211returned from object 212. Photodetector 216 may convert the laser light(e.g., returned laser beam 211) collected by lens 214 into an electricalsignal 218 (e.g., a current or a voltage signal). The current isgenerated when photons are absorbed in the photodiode. In someembodiments of the present disclosure, photodetector 216 may include anavalanche photodiode (APD), such as a single photon avalanche diode(SPAD), a SPAD array, or a silicon photo multiplier (SiPM).

Although scanner 210 is described as part of transmitter 202, it isunderstood that in some embodiments, scanner 210 can be part of receiver204, e.g., before photodetector 216 in the light path. The inclusion ofscanner 210 in receiver can ensure that photodetector 216 only captureslight, e.g., returned laser beam 211 from desired directions, therebyavoiding interferences from other light sources, such as the sun and/orother LiDAR systems. By increasing the aperture of mirror assembly inscanner 210 in receiver 204, the sensitivity of photodetector 216 can beincreased as well.

As described above, the incident angle of incident laser beam 207 may befixed relative to scanner 210, and the scanning of laser beam 209 may beachieved by rotating a single micro mirror or an array of micromachinedmirror assembly in scanner 210. FIG. 3A illustrates a schematic diagramof an exemplary micromachined mirror assembly 300, according toembodiments of the disclosure. FIG. 3B illustrates a top perspectiveview of micromachined mirror assembly 300 in FIG. 3A, according toembodiments of the disclosure. Different from some micromachined mirrorassemblies having fixed resonant frequencies that cannot be adjustedduring the operation, the operational resonant frequency ofmicromachined mirror assembly 300 can be adjusted online during theoperation, thereby allowing resonant frequency compensation due totemperature variation and/or enabling resonant frequency match amongmultiple micromachined mirror assemblies (e.g., in an array) due tofabrication process variation.

As illustrated in FIGS. 3A-3B, micromachined mirror assembly 300 mayinclude a micro mirror 302 and a pair of first and second suspendedbeams 304 and 306 mechanically connected to micro mirror 302 along anaxis 303 of micro mirror 302. Micro mirror 302 may be configured to tiltaround axis 303 as suspended beams 304 and 306 rotate due to the rigidjoint between micro mirror 302 and suspended beams 304 and 306. In someembodiments, micro mirror 302 and suspended beams 304 and 306 are formedusing MEMS microfabrication techniques from a same rigid semiconductorstructure, such as a silicon wafer. Micro mirror 302 may be covered by areflective layer disposed on its top surface (facing incident laserbeam). The reflective layer may be reflective to an incident laser beam,which is reflected by micromachined mirror assembly 300 to form areflected laser beam. By tilting micro mirror 302, the incident laserbeam may be reflected to a different direction, i.e., to form anotherreflected laser beam. It is understood that although micro mirror 302 isin a rectangle shape as shown in FIGS. 3A-3B, it is understood that theshape of micro mirror 302 is not limited to a rectangle shape and mayvary in other examples, such as a square, round, or eclipse shape.

Micromachined mirror assembly 300 may further include a pair of firstand second anchors 308 and 310 each mechanically coupled to a respectiveend of suspended beam 304 or 306 that is farther away from micro mirror302 and along axis 303. The other end of suspended beam 304 or 306 ismechanically coupled to micro mirror 302. Each one of anchors 308 and310 is affixed on a base (not shown) of micromachined mirror assembly300, according to some embodiments. Anchors 308 and 310 may be affixedto the base as both are formed using MEMS microfabrication techniquesfrom a same rigid semiconductor structure, such as a silicon wafer ormay be joined together using thermal bonding, adhesive bonding, orsoldering. Each one of suspended beams 304 and 306 is suspended from thebase, i.e., leaving a space therebetween, to allow certain movement(e.g., rotation and/or displacement) of suspended beams 304 and 306 withrespect to the base and anchors 308 and 310. In some embodiments, eachone of suspended beams 304 and 306 is configured to tilt around axis303, thereby driving the rotation of micro mirror 302. In someembodiments, each one of suspended beams 304 and 306 is made of a rigidmaterial, such as silicon, with substantially zero displacement in adirection along axis 303 (i.e., the axial direction).

As illustrated in FIGS. 3A-3B, micromachined mirror assembly 300 mayfurther include a pair of first and second actuators 312 and 314mechanically coupled to pair of suspended beams 304 and 306,respectively. A first actuator 312 may be configured to apply a firsttorsional stress around axis 303 to a first suspended beam 304, and asecond actuator 314 may be configured to apply a second torsional stressaround axis 303 to a second suspended beam 306. Consistent with someembodiments of the present disclosure, the first torsional stress andsecond torsional stress have different magnitudes, resulting in amagnitude difference between the stresses. In other words, first andsecond actuators 312 and 314 can create unbalanced torsional stresses onsuspended beams 304 and 306 on different sides of micro mirror 302 alongaxis 303. The unbalanced torsional stresses (i.e., a torsion) can betranslated into a tensional stress in the axial direction, which canincrease the resonant frequency of micro mirror 302. In someembodiments, the tensional stress caused by the torsion is maintainedduring the operation of micromachined mirror assembly 300, therebytuning the operational resonant frequency of micro mirror 302 from itsinitial resonant frequency. It is understood that the direction oftorsion may not affect the increase of the operational resonantfrequency of micro mirror 302. That is, the first torsional stressapplied by first actuator 312 may be greater than the second torsionalstress applied by second actuator 314, or vice versa.

As illustrated in FIGS. 3A-3B, first and second actuators 312 and 314may be electrostatic actuators, such as a set of comb drives.Electrostatic actuators rely on the force between two conductingelectrodes when a voltage is applied between them. Depending on thearrangement of the electrodes, various types of electrostatic actuatorsare possible, such as comb drive electrostatic actuators, parallel plateelectrostatic actuators, rotational electrostatic actuators cantileverelectrostatic actuators, to name a few. For example, as shown in FIG.3B, first actuator 312 may be an electrostatic comb drive actuator thatincludes a moveable comb 320 fixed to first suspended beam 304 and apair of fixed combs 322 and 324 fixed to the base on different sides offirst suspended beam 304. First suspended beam 304 and moveable comb 320may be arranged in a plane above fixed combs 322 and 324. Byalternatingly applying a voltage signal to the pair of fixed combs 322and 324, first suspended beam 304 and moveable comb 320 can tilt aroundaxis 303. Similarly, second actuator 314 may be an electrostatic combdrive actuator that includes moveable comb 326 fixed to second suspendedbeam 306 and a pair of fixed combs 328 and 330 fixed to the base ondifferent sides of second suspended beam 306. Second suspended beam 306and moveable comb 326 may be arranged in a plane above fixed combs 328and 330. By alternatingly applying a voltage signal to the pair of fixedcombs 328 and 330, second suspended beam 306 and moveable comb 326 cantilt around axis 303 as well.

In some embodiments, the unbalanced torsional stresses created by firstand second actuators 312 and 314 are achieved by applying two differentAC voltages V1 and V2 to first and second actuators 312 and 314,respectively. The difference between AC voltages V1 and V2 may beconverted into the magnitude difference of the first and secondtorsional stresses by a pair of electrostatic actuators, such as twosets of comb drives, as shown in FIGS. 3A-3B. The difference between ACvoltages V1 and V2 may be created in any suitable ways, such as byintroducing a DC offset (DC bias) or a phase offset (phase shift). Forexample, as shown in FIG. 4A, a DC offset Av may be applied to one ofthe two AC voltages, such as V1, to cause the difference of voltagemagnitude between V1 and V2 at each time point. In another example asshown in FIG. 4B, a phase offset may be applied to one of the two ACvoltages, such as V2, to cause a phase shift in V2 relative to V1. Thephase shift in turn can cause the difference of voltage magnitudebetween V1 and V2 at each time point. In some embodiments, thedifference of voltage magnitude between V1 and V2 is maintained to be atsubstantially the same level at each time point to maintain a constantoperational frequency increase during the entire operation cycle. Thecontrol of voltage signals V1 and V2 may be achieved by a controller(not shown) operatively coupled to first and second actuators 312 and314 to create and maintain the operational frequency increase as thedesired level.

It is understood that the type of electrostatic actuators for creatingunbalanced torsional stresses is not limited to comb drive actuators andcan include any other suitable electrostatic actuators, such as parallelplate electrostatic actuators, rotational electrostatic actuators, orcantilever electrostatic actuators, to name a few. It is also understoodthat the type of actuators for creating unbalanced torsional stresses isnot limited to electrostatic actuators and can include any othersuitable actuators, such as piezoelectric actuators, electromagneticactuators, thermal actuators, etc.

As described above, the torsion induced by first and second actuators312 and 314 can increase the resonant frequency of micro mirror 302 fromits initial resonant frequency. In some embodiments, to allow tuning ofthe operational resonant frequency of micro mirror 302 in both ways,i.e., increase and decrease, heating elements 316 and 318 may beincluded as part of micromachined mirror assembly 300. Heating elements316 and 318 may be disposed under and thermally coupled to both firstand second suspended beams 304 and 306, respectively (as shown in FIG.3A), or may be disposed under and thermally coupled to just one of firstand second suspended beams 304 and 306. During the operation ofmicromachined mirror assembly 300, heating elements 316 and 318 can heatfirst suspended beam 304 and/or second suspended beam 306 to increasethe temperature thereof. The resulting thermal expansion of firstsuspended beam 304 and/or second suspended beam 306 can cause thedecrease of the operational resonant frequency of micro mirror 302. Insome embodiments, a controller (not shown) is configured to dynamicallytune the operational resonant frequency of micro mirror 302 by adjustingthe voltage signals V1 and V2 applied to first and second actuators 312and 314 and/or adjusting the temperature of heating elements 316 and318.

In addition to indirectly translating a torsion into the tensionalstress, another way to introducing tensional stress for increasingresonant frequency of a micro mirror is to directly apply a tensionalstress through one or two suspended beams along the axis of the micromirror. For example, FIG. 5 illustrates a schematic diagram of anotherexemplary micromachined mirror assembly 500, according to embodiments ofthe disclosure. Similar to micromachined mirror assembly 300 describedabove in FIGS. 3A-3B, micromachined mirror assembly 500 also includes amicro mirror 502, a first suspended beam 504 and a second suspended beam506 each mechanically coupled to micro mirror 502 along an axis 503 ofmicro mirror 502, and a first anchor 508 and a second anchor 510 eachfixed on the base of micromachined mirror assembly 500 and mechanicallycoupled to a respective end of first or second suspended beam 504 or506. In some embodiments, micromachined mirror assembly 500 furtherincludes a first torsional actuator 512 mechanically coupled to firstsuspended beam 504 and configured to apply a first torsional stressaround axis 503 to first suspended beam 504, and a second torsionalactuator 514 mechanically coupled to second suspended beam 506 andconfigured to apply a second torsional stress around axis 503 to secondsuspended beam 506. The first torsional stress and second torsionalstress may be different to create a torsion that can be translated intoa tensional stress in the axial direction, as described above withrespect to micromachined mirror assembly 300. The details of micromirror 502, first and second suspended beams 504 and 506, first andsecond anchors 508 and 510, and first and second torsional actuators 512and 514 of micromachined mirror assembly 500 have been described abovewith respect to their counterparts of micromachined mirror assembly 300in FIGS. 3A-3B and thus, are not repeated.

As shown in FIG. 5, micromachined mirror assembly 500 further includes apair of tensional actuators 516 and 518 mechanically coupled to an endof respective first and second suspended beams 504 and 506 andconfigured to apply a tensional stress along axis 503 to first andsecond suspended beams 504 and 506. The tensional stress can be applieddirectly by tensional actuators 516 and 518 in the axial direction awayfrom micro mirror 502 to increase the operational frequency of micromirror 502. It is understood that in some embodiments, instead of havingtensional actuators 516 and 518 on both sides of micro mirror 502 asshown in FIG. 5, only one of tensional actuators 516 and 518 is kept onone side of micro mirror 502, i.e., mechanically coupled to one ofsuspended beams 504 and 506, which still can apply a tensional stressalong axis 503. Tensional actuators 516 and 518 can be any suitableactuators that can apply a tensional stress to first and secondsuspended beams 504 and 506 in the axial direction away from micromirror 502, i.e., pulling first and second suspended beams 504 and 506away from micro mirror 502 to increase the tension axially withinmicromachined mirror assembly 500, and thus, increase the resonantfrequency of micro mirror 502 during its operation. Tensional actuators516 and 518 can be, for example, electrostatic actuators, piezoelectricactuators, electromagnetic actuators, thermal actuators, etc.

For example, FIG. 6A illustrates a schematic diagram of a design ofmicromachined mirror assembly 500 in FIG. 5, according to embodiments ofthe disclosure. FIG. 6B illustrates a schematic diagram of anotherdesign of micromachined mirror assembly 500 in FIG. 5, according toembodiments of the disclosure. In both examples, a tensional actuator,such as one or more sets of comb drive electrostatic actuators, isdisposed on one side of micro mirror 502 to apply a tensional stressalong axis 503 of micro mirror 502 to increase the operational resonantfrequency of micro mirror 502. It is understood that in otherembodiments, another tensional actuator may be similarly disposed on theother side of micro mirror 502 as well.

In one design as shown in FIG. 6A, a set of comb drives 602 ismechanically coupled to an end of second suspended beam 506 andconfigured to apply a tensional stress along axis 503 to secondsuspended beam 506. In some embodiments, set of comb drives 602 includea fixed comb 604 fixed to second anchor 510 that does not move relativeto the base of micromachined mirror assembly 500, and a movable comb 606fixed to second suspended beam 506 that is movable along the axialdirection. By applying a voltage to set of comb drives 602, movable comb606 can be attracted by fixed comb 604 toward second anchor 510 andthus, create a tensional stress to second suspended beam 506 in theaxial direction away from micro mirror 502. It is understood that fixedcomb 604 may not be fixed to anchor 510 in some embodiments. In someembodiments, fixed comb 604 is electrically separated from the rest ofcomponents (e.g., micro mirror 502, first and second suspended beams 504and 506, first and second anchors 508 and 510, and movable comb 606) inmicromachined mirror assembly 500.

In another design as shown in FIG. 6B, two sets of comb drives 610 and616 each is mechanically coupled to a respective end of a firstsub-suspended beam 608 and a second sub-suspended beam 614. First andsecond sub-suspended beams 608 and 614 may be mechanically coupled to aconnection point of second suspended beam 506 in a certain angle to formrigid joints, such that tensional stresses applied to first and secondsub-suspended beams 608 and 614 can be transferred to second suspendedbeam 506, which can be in turned transferred to micro mirror 502 alongaxis 503 of micro mirror 502 to increase the operational resonantfrequency of micro mirror 502. In some embodiments, the connection pointis close to anchor 510 to minimize the rotation of first and secondsub-suspended beams 608 and 614 around axis 503. Similar to set of combdrive 602 described above with respect to FIG. 6B, each set of combdrives 610 or 616 may include a movable comb drive fixed to respectivefirst sub-suspended beam 608 or second sub-suspended beam 614, and afixed comb drive fixed to a respective anchor 612 or 618. By applying avoltage to each set of comb drives 610 or 616, each movable comb can beattracted by the respective fixed comb toward anchor 612 or 618 andthus, create a tensional stress to respective first sub-suspended beam608 or second sub-suspended beam 614, which can be in turned transferredto second suspended beam 506 along axis 503.

Referring back to FIG. 5, in some embodiments, to allow tuning of theoperational resonant frequency of micro mirror 502 in both ways, i.e.,increase and decrease, heating elements 520 and 522 may be included aspart of micromachined mirror assembly 500. Heating elements 520 and 522may be disposed under and thermally coupled to both first and secondsuspended beams 504 and 506, respectively (as shown in FIG. 5), or maybe disposed under and thermally coupled to just one of first and secondsuspended beams 504 and 506. During the operation of micromachinedmirror assembly 500, heating elements 520 and 522 can heat firstsuspended beam 504 and/or second suspended beam 506 to increase thetemperature thereof. The resulting thermal expansion of first suspendedbeam 504 and/or second suspended beam 506 can cause the decrease of theoperational resonant frequency of micro mirror 502. In some embodiments,a controller (not shown) is configured to dynamically tune theoperational resonant frequency of micro mirror 502 by adjusting thevoltage signals applied to first and second torsional actuators 512 and514, the voltage signals applied to first and second tensional actuators516 and 518, and/or adjust the temperature of heating elements 520 and522.

FIG. 7 illustrates a flow chart of an exemplary method 700 for driving amicromachined mirror assembly, according to embodiments of thedisclosure. For example, method 700 may be implemented by micromachinedmirror assemblies 300 and 500 described above. However, method 700 isnot limited to that exemplary embodiment. Method 700 may include stepsS702-S706 as described below. It is to be appreciated that some of thesteps may be optional to perform the disclosure provided herein.Further, some of the steps may be performed simultaneously, or in adifferent order than shown in FIG. 7.

In step S702, a resonant frequency of a micromachined mirror assembly isset at an initial value. The initial value may be pre-set by the designand fabrication process of the micromachined mirror assembly. In someembodiments, the initial value is pre-set as the minimum value, whichcan be increased to a higher value during the operation by increasingthe resonant frequency during the operation of the micromachined mirrorassembly. In some embodiments, the initial value is pre-set as themaximum value, which can be decreased to a lower value during theoperation by decreasing the resonant frequency during the operation ofthe micromachined mirror assembly.

In step S704, a tensional stress is applied along an axis of themicromachined mirror assembly to increase the resonant frequency to afirst operational value greater than the initial value during theoperation of the micromachined mirror assembly. In some embodiments, thetensional stress is directly applied to one or two suspended beams ofthe micromachined mirror assembly by one or more tensional actuators. Insome embodiments, the tensional stress is indirectly transferred fromunbalanced torsional stresses applied to two suspended beams of themicromachined mirror assembly by a plurality of torsional actuators. Insome embodiments, two different voltages are applied to twoelectrostatic actuators on different sides of the micro mirror of themicromachined mirror assembly, respectively, to create two torsionalstresses with different magnitudes. In some embodiments, the initialvalue of the resonant frequency is pre-set as the minimum value, suchthat the operational resonant frequency is increased to the desiredfirst operational value by applying a suitable level of tensional stressalong the axis of the micromachined mirror assembly.

In step S706, heat is applied to the micromachined mirror assembly todecrease the resonant frequency to a second operational value smallerthan the initial value during the operation of the micromachined mirrorassembly. In some embodiments, heating elements are thermally coupled toone or two suspended beams of the micromachined mirror assembly to heatthe suspended beam(s), thereby decreasing the second operational value.In some embodiments, the initial value of the resonant frequency ispre-set as the maximum value, such that the operational resonantfrequency is increased to the desired second operational value byapplying a suitable level of heat to the micromachined mirror assembly.

It is understood that in some embodiments, the initial value of theresonant frequency of the micromachined mirror assembly is pre-set atneither the maximum value nor the minimum value, and step S704 and stepS706 can be both performed in any suitable times and order todynamically tune the initial value to a desired operational value inboth ways.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed system andrelated methods. Other embodiments will be apparent to those skilled inthe art from consideration of the specification and practice of thedisclosed system and related methods.

It is intended that the specification and examples be considered asexemplary only, with a true scope being indicated by the followingclaims and their equivalents.

What is claimed is:
 1. A micromachined mirror assembly, comprising: amicro mirror configured to tilt around an axis; a first suspended beamand a second suspended beam each mechanically coupled to the micromirror along the axis; a first actuator mechanically coupled to thefirst suspended beam and configured to apply a first torsional stressaround the axis to the first suspended beam; and a second actuatormechanically coupled to the second suspended beam and configured toapply a second torsional stress around the axis to the second suspendedbeam, wherein the first torsional stress and second torsional stress areunbalanced by a magnitude difference.
 2. The micromachine mirrorassembly of claim 1, wherein the first actuator is a first electrostaticactuator, and the second actuator is a second electrostatic actuator. 3.The micromachine mirror assembly of claim 2, wherein the firstelectrostatic actuator comprises a first set of comb drives, and thesecond electrostatic actuator comprises a second set of comb drives. 4.The micromachine mirror assembly of claim 2, wherein a first AC voltageapplied to the first electrostatic actuator is different from a secondAC voltage applied to the second electrostatic actuator.
 5. Themicromachine mirror assembly of claim 4, wherein the first and second ACvoltages have a DC offset.
 6. The micromachine mirror assembly of claim4, wherein the first and second AC voltages have a phase offset.
 7. Themicromachine mirror assembly of claim 1, wherein the magnitudedifference between the first torsional stress and the second torsionalstress is translated into a tensional stress along the axis.
 8. Themicromachine mirror assembly of claim 7, wherein a resonant frequency ofthe micro mirror is increased by the tensional stress along the axis. 9.The micromachine mirror assembly of claim 8, further comprising at leastone heating element thermally coupled to at least one of the first andsecond suspended beams and configured to apply heat to the at least oneof the first and second suspended beams, such that the resonantfrequency of the micro mirror is decreased.
 10. A micromachined mirrorassembly, comprising: a micro mirror configured to tilt around an axis;a first suspended beam and a second suspended beam each mechanicallycoupled to the micro mirror along the axis; and at least one tensionalactuator mechanically coupled to an end of at least one of the first andsecond suspended beams and configured to directly apply a tensionalstress along the axis in a longitudinal direction away from the micromirror to the at least one of the first and second suspended beams. 11.The micromachined mirror assembly of claim 10, further comprising: afirst torsional actuator mechanically coupled to the first suspendedbeam and configured to apply a first torsional stress around the axis tothe first suspended beam; and a second torsional actuator mechanicallycoupled to the second suspended beam and configured to apply a secondtorsional stress around the axis to the second suspended beam.
 12. Themicromachine mirror assembly of claim 10, wherein the tensional actuatoris an electrostatic actuator.
 13. The micromachine mirror assembly ofclaim 12, wherein the electrostatic actuator comprises a set of combdrives.
 14. The micromachine mirror assembly of claim 10, wherein the atleast one tensional actuator comprises: a first tensional actuatormechanically coupled to an end of the first suspended beam andconfigured to apply a first tensional stress along the axis to the firstsuspended beam; and a second tensional actuator mechanically coupled toan end of the second suspended beam and configured to apply a secondtensional stress along the axis to the second suspended beam.
 15. Themicromachine mirror assembly of claim 10, wherein the tensional stresspulls the at least one of the first and second suspended beams away fromthe micro mirror.
 16. The micromachine mirror assembly of claim 10,wherein a resonant frequency of the micro mirror is increased by thetensional stress along the axis.
 17. The micromachine mirror assembly ofclaim 16, further comprising at least one heating element thermallycoupled o at least one of the first and second suspended beams andconfigured to apply heat to the at least one of the first and secondsuspended beams, such that the resonant frequency of the micro mirror isdecreased.
 18. The micromachine mirror assembly of claim 10, whereineach of the first and second suspended beams is made of silicon.
 19. Amethod for driving a micromachined mirror assembly, comprising: settinga resonant frequency of the micromachined mirror assembly at an initialvalue; and directly applying a tensional stress along an axis of themicromachined mirror assembly in a longitudinal direction away from themicromachined mirror assembly to increase the resonant frequency to afirst operational value greater than the initial value during operationof the micromachined mirror assembly.
 20. The method of claim 19,further comprising applying heat to the micromachined mirror assembly todecrease the resonant frequency to a second operational value smallerthan the initial value during operation of the micromachined mirrorassembly.